Pipe unions and couplings are used to join together two sections of pipe and usually require a machined metal to metal or a resilient seal to prevent leakage and loss of fluid at the coupling. In the prior art, O-rings or gaskets fashioned out of neoprene, or other flexible plastics or rubber, are almost exclusively used to seal these couplings. These gaskets or seals are sometimes held in position on one of the two sealing members by the use of a groove or slot. Other gasketed unions use only the pressure of the two sealing members to hold the seal in place. The union is joined together with the second member and the seal compressed between the trough and the opposite face of the second member with threads or other structure used to tighten the coupling and compress the seal. An example of a typical prior art device is disclosed in Callahan U.S. Pat. No. 3,288,494.
Problems have been experienced with the prior art devices in that the O-ring seal or gasket is dependent upon the pressure created in the mechanical tightening of the coupling to ensure a good seal. This tight mechanical connection compresses the O-ring or gasket and distorts it around the small base existing between the machined surfaces at which the seal takes place. In the gasketed union there is a common tendency for the gasket material to either cold flow, migrate or take a compressive set causing leakage. The metal-to-metal union is largely dependent on a meshing of two highly polished or ground surfaces to effect a seal. Any small scratches or knicks on the sealing surface will interfere with the proper meshing of the two surfaces and cause leakage. These surface imperfections are common due to handling during assembly and are often incurred during disassembly. It is usually the practice that the two sealing surfaces in the metal-to-metal union are ground to match each other. This does not allow the replacement of just one side of the union.
These problems are particularly troublesome in applications subjecting the pipe union to the extremes of temperature and moisture, such as in a steam line. The added stress caused by these extremes results in a substantially reduced seal life and the increased expense of seal replacement much earlier than should be required.
Applicant has succeeded in inventing a pipe union and seal combination which overcomes the problems existing in the prior art and which increases both the operating life and the operating temperature ranges of the pipe coupling and seal. The first embodiment of applicant's invention generally includes the use of a compound seal having a flexible U-shaped shell with a central spring disposed within the arms of the "U". Seals such as are manufactured and sold under the trade name "Omniseal" are suitable for use in applicant's invention. The U-shaped shell may be constructed of any type of suitable plastic including polytetrafluoroethylene, TEFLON.TM., or other similar plastics. The central spring may be a helical wound flat wire spring and is disposed within the arms of the U-shaped shell to provide it shape, add to its natural resiliency, and compensate for the cold flow properties of the shell material. This compound seal is much more resistant to the extremes of temperature and pressure than was available with the gasketed, O-ring or metal-to-metal unions in the prior art, and dramatically increases the ability to reliably reuse or remake the union without leakage.
Applicant's compound seal is tightened not only through the mechanical pressure created in the coupling itself but also by the pressure of the fluid flowing through the pipe union. Furthermore, as the outer shell is preferably constructed of plastic or other soft materials, scratches or other damage to the sealing surfaces of both the metal glands or the seal itself are compensated for because upon installation, the spring-imposed load causes the shell material to cold flow or migrate into discontinuities in the sealing surface. Cold flow, often considered a disadvantage of TFE type plastics, is utilized to provide conformance with the sealing surface, ensuring positive sealing. Cold flow stops when a point of equilibrium is reached between the load of the spring and the compressive yield of the TFE. Ample squeeze remains to provide long-term sealing.
A second embodiment of applicant's invention includes a metal C-ring having its opening facing the center of the union and sized to have a cross-sectional width greater than the trough within which it is inserted. A sealing surface of each part of the union is brought into engagement with the C-ring and compresses the C-ring as it is assembled. Generally, the C-ring is made of a hardened metal such as stainless steel with a coating of a somewhat softer metal such as silver. Thus, a sealing surface is formed between the stainless steel of the C-ring and the union with a buffer of a softer metal such as silver. Although this softer surface on the metal C-ring is susceptable to scratching, the metal C-ring is capable of operating at temperatures ranging from cryogenic to 1200.degree. F. Thus, applicant's invention is suitable for use in applications requiring the piping of cryogenic to superheated fluids under temperatures which would destroy conventional seals. Those applications which would not require a constant opening and closing of the pipe union but instead could be fixed permanently in place until a failure occurred would be particularly suited for use with this embodiment of applicant's invention.
These and other advantages of applicant's invention may be more fully understood by referring to the drawings and detailed description of the preferred embodiment.